1. Field of the Invention
The invention is a process for the electroless deposition of metals on nonmetallic surfaces. It involves in particular the sensitization and activation of the nonmetallic surface for electroless metal disposition.
2. Description of the Prior Art
Electroless deposition of metals on nonmetallic surfaces (i.e., polymer surfaces and ceramic surfaces) has become of increasing importance in recent years. An increasingly large variety of electronic circuits is now manufactured on printed wiring boards made up of nonmetallic surfaces which have been metallized to provide conducting paths. Electroless deposition of metals on nonmetallic surfaces is also used for decorative plating on such articles as automobile parts, consumer appliances, etc. Inexpensive and reliable processes for carrying out such electroless plating procedures are in great demand. It is highly desirable from an economic point of view to develop inexpensive, simple and highly reliable processes for electroless plating of metals on nonmetallic surfaces. For applications relating to the fabrication of electronic circuits good control over the deposition process is often required. For example, rapid and uniform initiation of electroless deposition assures more uniform deposition of metal which is highly advantageous for printed circuit boards to be used in electronic circuit fabrication.
An important part of the process for electroless deposition of metals on nonmetallic surfaces is the preparation of the nonmetallic surface to make it catalytic for electroless deposition. A variety of processes is in use for making the nonmetallic surface catalytic for electroless deposition. Particularly important is the so-called two-step process in which the nonmetallic surface is first exposed to a sensitizer solution (typically SnCl.sub.2) and then in a separate step exposed to an activator solution (typically PdCl.sub.2). The two-step process often gives superior results for particular surfaces and electroless deposition solutions and is preferred where a sensitizer solution is used for pattern delineation. A review of catalytic activation procedures using the two-step process may be found in a book entitled Metallic Coating of Plastics by William Goldey, Electrochemical Publications Ltd. 1968, Volume 1, particularly Chapter 5.
The increased sophistication of electronic circuits has increased the demands for more precise control of the electroless deposition process. Particularly important are uniform thickness and more precise pattern delineation. Attainment of these goals is aided by reducing or eliminating initiation times usually evident in electroless deposition processes. Additionally, decreased wash-in of sensitizer and activator materials into the electroless plating bath is highly desirable, as these substances have deleterious effects on bath stability.